Apparatus for continuous slag treatment of silicon

ABSTRACT

The invention relates to an apparatus for continuous slag treatment of molten silicon for removal of one a more impurity elements from the silicon, the apparatus comprising a vessel ( 1 ) intended to contain molten silicon and liquid slag. The vessel ( 1 ) has an upwardly extending overflow ( 4 ) for slag connected to an outlet opening ( 3 ) for slag is the bottom ( 2 ) of the vessel, a open-ended pipe ( 5 ) arranged about and at a distance from the overflow ( 4 ) such than there is an annulus between the pipe ( 5 ) and the overflow ( 4 ), which pipe ( 5 ) extends upwards to a level above the top of the overflow ( 4 ) and downwards to the bottom ( 2 ) of the vessel, the pipe ( 5 ) having at least one opening ( 6 ) at the bottom of the vessel ( 1 ) a closeable outlet opening ( 8,9 ) for treated silicon in the sidewall of the vessel, means ( 14 ) for supply of heat energy to the vessel, means for intermittent supply of solid or molten silicon to vessel and means for continuous of substantially continuous supply of solid or liquid slag to the top of the vessel.

FIELD OF INVENTION

The present invention relates to an apparatus for continuous slagtreatment of molten silicon.

BACKGROUND ART

From Norwegian patent No. 180532 it is known a method for removingimpurities, particularly boron, from molten silicon by continuoustreatment of the silicon with slag. According to Norwegian patent No.180532 the slag is added continuously or substantially continuously tothe molten silicon and the slag is inactivated or removed continuouslyor substantially continuously as soon as equilibrium is reached betweenthe slag and the molten silicon with respect to the impurity element orelements which are to be removed.

If the slag has a higher density than molten silicon, it is proposed inNorwegian patent No. 180532 to supply the slag to the top of the siliconbath and continuously or substantially continuously withdraw the slagfrom the bottom of the vessel in which the treatment is carried out.

It has however been found that it is difficult to withdraw the slagcontinuously or substantially continuously from the bottom of the vesselwithout withdrawing a substantially amount of silicon together with theslag. Further is has been found that it is difficult to remove all slagfrom the silicon. The yield of silicon by use of the method disclosed inNorwegian patent No. 180532 will for these reasons be low and therefined silicon can be contaminated with some slag particles.

DISCLOSURE OF INVENTION

By the present invention an apparatus has been provided where slag canbe withdrawn continuously without any silicon following the slag andwhere it can be obtained a refined silicon having a very low slagcontent.

The present invention thus relates to an apparatus for continuous slagtreatment of molten silicon for removal of one a more impurity elementsfrom the silicon, the apparatus comprising a vessel intended to containmolten silicon and liquid slag, the vessel having an upwardly extendingoverflow for slag connected to an outlet opening for slag is the bottomof the vessel, a open-ended pipe arranged about and at a distance fromthe overflow such than there is an annulus between the pipe and theoverflow, which pipe extends upwards to a level above the top of theoverflow and downwards to the bottom of the vessel, the pipe having aleast one opening at the bottom of the vessel, a closeable outletopening for treated silicon in the sidewall of the vessel, means forsupply of heat energy to the vessel, means for intermittent supply ofsolid or molten silicon to vessel and means for continuous orsubstantially continuous supply of solid or liquid slag to the top ofthe vessel.

The vessel preferably has a circular cross-section, but may also have anoval, square or rectangular cross-section.

The overflow is preferably cylindrical, but can also have oval, squareor rectangular cross-section.

The vessel preferably has a gas tight sealed lid having openings forsupply of silicon and slag. The gas tight lid makes it possible tocontrol the atmosphere above the molten bath in the vessel.

The means for supply of heat energy to the vessel is preferably aninduction heating device or a resistance heating device arranged on theoutside of the vessel. However, the means for supply of heat energy canalso be a plasma burner or electrodes arranged in the vessel.

It has been found that by the use of the apparatus according to thepresent inventions where slag is removed by overflow, a very goodremoval of slag from the silicon is obtained and thereby a higher yieldof silicon is obtained. Further, silicon which is tapped from the vesselafter the slag treatment will not contain slag particles as the outletopening for the treated silicon is arranged above the level of slag inthe vessel. The use of the apparatus according to the invention furthergives an increased control of the volume flows in the vessel as it canbe controlled that the amount of slag supplied to the vessel is equal tothe slag tapped from the vessel via the overflow.

SHORT DESCRIPTION OF DRAWINGS

FIG. 1 is a vertical cut through the apparatus according to theinvention, and

FIG. 2 is a horizontal cut taken along line A—A in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

On FIGS. 1 and 2 it is shown a cylinder-shaped vessel 1. The vessel 1 ismade from graphite. The vessel 1 has an outlet opening 3 for slag in thebottom 2. The outlet opening 3 communicates with an overflow 4 whichextends from the bottom 2 of the vessel 1 and upwardly in the vessel 1.In the embodiment shown in the Figures the overflow 4 has a circularcross-section, but an overflow having other cross-sections such as oval,square or rectangular cross-sections can also be used. About theoverflow 4 it is arranged an open-ended pipe 5 extending from the bottom2 of the vessel 1 and upwardly to a level above the upper end of theoverflow 4. The pipe 5 shown in FIGS. 1 and 2 has a circularcross-section, but may have other cross-sections such as oval,rectangular or square. At the bottom of the vessel 1 the pipe 5 hasopenings 6 allowing slag to flow from the space between the inside ofthe vessel 1 and the outside of the pipe 5 and into the annulus 7between the inside of the pipe 5 and the outside of the overflow 4. Theopenings 6 may be horizontal of they can be inclined towards thehorizontal. In the sidewall of the vessel 1 at a level above the bottom2 of the vessel 1, there is arranged a closeable opening 8 for tappingof treated silicon, which opening 8 communicates with a tapping channel9 arranged in the sidewall of the vessel 1. The opening 8 can be openedand closed by means of a vertically arranged stopper rod 10. For openingof the opening 8 the stopper rod 10 is moved upwardly and for closing ofthe opening 8 the stopper rod 10 is moved downwardly.

The vessel 1 has a gas tight lid 11 with openings 12, 13 for supply ofsilicon to be treated and for continuous supply of solid or liquid slag.

Heat energy is supplied to the apparatus shown in FIGS. 1 and 2 by meansof an induction heating means 14. Alternatively the supply of heatenergy can be done by means of electric heating elements arranged aboutthe circumference of the vessel 1.

Both the overflow 4 and the pipe 5 are made from a material which isresistant against liquid slag and molten silicon. Preferably all theparts of the apparatus are made from graphite, but other materials suchas silicon carbide, silicon nitride, silicon oxynitride or graphitehaving a layer of silicon carbide, silicon nitride or silicon oxynitridecan also be used.

Even if the embodiment in the Figures shows a cylinder-shaped vessel 1,it is within the scope of the invention to use a vessel 1 having othercross-sections such s oval, rectangular or square cross-section.

By use of the apparatus according to the present invention, solid amolten silicon to be treated is supplied to the vessel 1 in the spacebetween the inside of the vessel 1 and the outside of the pipe 5 and thevessel 1 with its content is heated to a temperature above the meltingpoint of silicon by means of the induction heating means 14. Thereafterslag, preferably in liquid form, is continuously or substantiallycontinuously supplied to the vessel 1 through the openings 12 and 13 inthe lid 11. The slag supplied has a higher density then the moltensilicon.

The slag will when supplied, sink down through the bath of moltensilicon and contaminating elements in the silicon will be transferred tothe slag. Since the slag has a higher density than the silicon, the slagwill gradually form a slag layer on the bottom of the vessel 1 and flowthrough the openings 6 in the pipe 5 and into the annulus between theoutside of the overflow 4 and the inside of the pipe 5. When asufficient amount of slag has been supplied a stationary situation willbe reached. When a further amount of slag is supplied, an equal amountof slag will flow into the overflow 4 and out through the outlet opening3 for slag. This situation is indicated in FIG. 1 where the top of thesilicon bath is indicated by reference numeral 20 and where the bottomof the silicon bath is indicated by reference numeral 21, while the slagthat has sunk through the silicon bath is below the silicon bath andfills the annulus between the overflow 4 and the pipe 5. Slag will thusbe continuously supplied and continuously removed.

When the molten silicon bath has obtained a sufficient purity, thesupply of slag is stopped, and when all slag has sunk through thesilicon bath, the outlet opening 8 is opened by lifting the stopper rod10. The part of the silicon bath situated above the outlet opening 8will then flow out of the vessel 1 through the channel 9 and iscollected in a ladle (not shown) under the channel 9. Thereafter theoutlet opening 8 is closed by moving the stopper rod 10 downwardly. Anew batch of silicon is then supplied to the vessel 1 and the continuoussupply of slag is again started.

By arranging the outlet opening 8 a distance above the upper level ofthe slag layer, slag particles in the silicon tapped from the vessel 1is prevented.

1. In an apparatus for continuous slag treatment of molten silicon forremoval of one a more impurity elements from the silicon, the apparatuscomprising a vessel intended to contain molten silicon and liquid slagthe vessel has an upwardly extending overflow for slag connected to anoutlet opening for slag is the bottom of the vessel, and means forsupplying heat energy to the vessel, the improvement comprising: anopen-ended pipe arranged about and at a distance from the overflow suchthat there is an annulus between the pipe and the overflow, which pipeextends upwards to a level above the top of the overflow and downwardsto the bottom of the vessel, the pipe having at least one opening at thebottom of the vessel, a closeable outlet opening for treated silicon inthe sidewall of the vessel, and means for intermittent supplying solidor molten silicon to the vessel and means for continuous orsubstantially continuous supplying solid or liquid slag to the top ofthe vessel.
 2. The apparatus according to claim 1, wherein the vesselhas a circular, oval, square or rectangular cross-section.
 3. Theapparatus according to claim 1, wherein the overflow has a circular,oval, square or rectangular cross-section.
 4. The apparatus according toclaim 1 wherein the pipe has a circular, oval, rectangular or squarecross-section.
 5. The apparatus according to claim 1 wherein the vessel,the overflow and the pipe are made from graphite.
 6. The apparatusaccording to claim 1 wherein the vessel, the overflow and the pipe aremade from graphite having a surface layer of silicon carbide, siliconnitride or silicon oxynitride.
 7. The apparatus according to claim 1wherein the vessel, the overflow and the pipe are made from siliconcarbide, silicon nitride or silicon oxynitride.
 8. The apparatusaccording to claim 1, wherein the vessel has a gas tight lid withopenings for supply of silicon and slag.
 9. The apparatus according toclaim 1, wherein the means for supplying heat energy is an inductionheating means arranged on the outside of the vessel.
 10. The apparatusaccording to claim 1, wherein the means for supplying heat energy is aresistance heating means arranged on the outside of the vessel.